Final answer:
The greatest oxygen partial pressure in the blood is achieved by countercurrent gas exchange due to the optimal maintenance of a gradient for gas exchange along the entire length of the exchange surface.
Step-by-step explanation:
The system that achieves the greatest oxygen partial pressure in the blood, all else being equal, is b. Countercurrent gas exchange. This is because countercurrent exchange allows for the maximum gradient to be maintained between the oxygen in the environment (such as water for fish) and the blood, which means that oxygen is able to diffuse from the water into the blood over the entire length of the exchange surface. In contrast, tidal gas exchange, as occurs in human lungs, does not maintain such a persistent gradient, and concurrent gas exchange will have areas where the gradient between the external environment and the blood is not optimally maintained for maximum gas exchange.
Oxygen and carbon dioxide flow from areas of high partial pressure to areas of low partial pressure during the gas exchange between the blood and the body's cells due to the principles established by Dalton's law, which states that the total pressure exerted by a mixture of gases is the sum of the pressures exerted independently by each gas in the mixture according to its volume in the mixture.